feat(ipc)!: remove the cache-key mailboxes (control socket stage 5)

The control socket is now the only way the web interface sends the display a
command. The display stops reading display_on_demand_request and
plugin_error_clear_request, and the web interface stops writing them.

- Display: no mailbox poll (MailboxWatch, the 1 s / 0.25 s cadence,
  _consume_on_demand_request, the deprecation log) and no persisted
  display_on_demand_processed_id guard; the error publisher reads no clear
  request. CacheManager.file_signature and MailboxWatch are removed.
- A write to either retired key is dropped by CacheManager.save_cache and
  logged once per writer, naming the plugin from the call stack (or the
  request's plugin_id), with the API to move to.
- Web: on-demand start with no display listening starts the service (when
  start_service) and sends the request again once the socket answers (45 s,
  10 s for a running service without a socket yet); every other failure is
  a 503 (400 for invalid_args). Stop answers 503 when no display listens,
  unless stop_service. errors/clear answers 503 with a reason-specific
  message instead of writing a request; clear_pending is always false.
  src.ipc.client.should_fall_back is replaced by display_not_listening.
- Kept: display_current_state, display_on_demand_state,
  plugin_runtime_snapshot and the heartbeat (read whenever the socket cannot
  answer), and display_on_demand_config (the display's resume record).

Tests: mailbox-only tests removed (test_on_demand_mailbox.py, the mailbox
cadence, file_signature and MailboxWatch tests); tests that injected
requests through the mailbox now use the socket queue or a plugin's
in-process request. The run-loop harness sends on-demand requests over its
fake control socket, so four golden traces change: on-demand starts and
stops land at the request instant instead of the next 0.25 s mailbox look
(one frame fewer on the screen they end), and in vegas.json within one
frame instead of 263 ms, which shifts the later 1 s-throttled WiFi-notice
check by under a second.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_ipc_contract.py

@@ -3,7 +3,7 @@
 Pure data, so every test here runs on every platform. What they pin:
 
 * a request and a response survive encode -> decode -> parse unchanged, and
-  the on-demand arguments carry exactly what the file mailbox carries;
+  the on-demand arguments carry exactly what the REST route sends;
 * the envelope and the arguments refuse what the display could not act on
   (missing ids, wrong types, a non-finite duration) with a stable error code;
 * framing never holds more than one message's worth of bytes, however the
@@ -184,8 +184,8 @@ class TestOnDemandArgs:
 
 
 class TestMailboxShape:
-    """Socket commands are handed to the mailbox's own handler, so they must
-    look exactly like what the web route writes to the mailbox."""
+    """Socket commands are handed to the display's on-demand handler, so they
+    must look exactly like the request dict it takes."""
 
     def test_start(self):
         args = OnDemandStartArgs(plugin_id='clock', mode='clock_main', duration=60.0,